We introduce a new statistical method, the Analysis of Skewness, for quantifying large-scale evolutionary trends as a combination of both passive and driven mechanisms. Our approach is based on the skewness of subclades within a parent clade. We partition the total skewness of the parent clade into three components: (1) skewness between subclades, (2) skewness within subclades, and (3) skewness due to changes in variance among subclades. The third component corresponds to a new type of passive mechanism, in which overall skewness of a parent clade is due to greater variability in subclades to the right of the mean. This partitioning allows us to quantitatively decompose the mechanisms underlying trends into two components: a driven portion and a passive portion. We thus quantify the effect of small-scale dynamics on large-scale behavior of clades. Applications are given to Miocene-Pliocene rodent size and Ordovician brachiopod muscle geometry.